Cylindrical radiant energy collectors are trough-like structures which concentrate incident radiant energy. The structure usually includes as a concentration means a reflecting wall or walls, or a lens. The wall or walls are formed by extending a given cross section along a longitudinal axis perpendicular to the cross section. They direct and concentrate, usually by reflection, incident energy onto a heat absorbing surface of an energy receiver. The absorber surface may be a duct with a coolant or a group of thermally interconnected ducts. Radiant energy directed onto the ducts is absorbed as heat by the ducts and is removed by the fluid made to flow through the ducts by pumping means, such as a pump or thermosiphon. In those cylindrical collectors where only a portion of the heat absorbing surface has energy directed upon it, at a particular instance, rather than having all of the heat absorbing surface of the absorber having energy directed thereupon, inefficiency results from heat losses from those hot receiver surfaces which do not have radiation directed thereon and therefore which do not absorb radiation. Heat flows to those surfaces by conduction through the metal receiver from those surfaces which do absorb radiation due to the thermal interconnected nature of the receiver, and by convection from the hot fluid circulating through heated and unheated portions of the receiver, since in practice the fluid even before removing heat from the receiver is hotter than the environment.
These problems are particularly evident in cylindrical imaging solar radiant energy collectors with an essentially parabolic reflecting wall. Such a collector is advantageous in that it requires no diurnal tracking. With solar rays coplanar with the axis plane about which the parabola defining the parabolic reflecting wall is symmetric, the image, i.e. the envelope of radiant energy directed by the reflecting wall and falling on the absorbing surface of the receiver positioned at the focus of the parabolic reflecting wall parallel to the longitudinal axis, is a very narrow strip or band. This image moves across the absorbing surface during the hours of solar radiation collection, so that only a portion of the absorbing surface is heated at any instant. With prior art cylindrical imaging collectors with a parabolic reflecting wall the absorbing surface is usually a single pipe or a group of side-by-side contacting pipes. Efficiency is degraded by the losses due to radiation and convection from those portions of the absorbing surface upon which the image is nonincident and from pumping fluid through those portions of the absorbing surface upon which the image is nonincident.
It is therefore an object of this invention to improve the efficiency of cylindrical radiant energy collectors.
Another object of this invention is to improve the efficiency of cylindrical imaging collectors with an essentially parabolic reflecting wall.
Another object of this invention is to reduce radiant heat loss from unheated portions of the absorbing surface of the energy receiver of a cylindrical radiant energy collector.
Another object of this invention is to control coolant flow through the energy receiver of a cylindrical radiant energy collector by limiting coolant flow to heated portions of the absorbing surface of the energy receiver of a cylindrical radiant energy collector.